The present invention relates to methods and apparatus for measuring the temperature of an object, in particular a semiconductor wafer during processing.
During the processing of a semiconductor wafer, it is required that the wafer be subjected to certain temperatures for different process steps. Although the temperature inside the oven might be known, the temperature of the wafer itself is the parameter that is critical. One method is to physically connect a thermocouple to the wafer to measure its temperature. However, a good connection is difficult to make unless the thermocouple is embedded in the wafer, which will make that portion of the wafer unusable.
One method, disclosed in U.S. Pat. No. 4,984,902, uses a pyrometer to measure the infrared emissivity of the wafer during heating. When the wafer is heated, it will emit an amount of infrared radiation of a particular wavelength band which can be detected by a pyrometer and correlated to the temperature of the wafer. This requires a calibration process in which a sample wafer of the batch to be processed has a thermocouple embedded in it so that the apparatus can be calibrated for the emissivity of that type of a wafer. Once calibrated, a run of wafers can be done without using an embedded thermocouple, relying solely on the emissions detected by the pyrometer. Unfortunately, the need to embed a thermocouple causes delays in the assessment of the wafers. Furthermore, inaccuracies in measurements can arise due to small variations in the emissive properties of the wafers. It is therefore desirable to have a method for measuring the actual temperature of the wafer without embedding a thermocouple and without dependence on the wafer's emissive properties.